1200a 3ph 4 wire 208/120v service

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bplus

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I have a comm bldg with florescent lighting.With a load calculation I figure I need a 1200a capacity.The runs are figured to be run under ground from Main sw to txmfr pad .I figure w/flour ltgs the neutral is current carrying and I have to use this in my derating ( w/=80%).The feeders are to be run in pvc and no more than 4 cond per conduit.Which translates to me as(4-400kcm (90 deg.C) w/3/0 grnd wire in ea of the 4-3" pvc conduits.
Because the conduits are run underground , does this negate any of the derating rules.
Secondary runs are under 30 feet.(VD ok)
I welcome any opinion or any alternate approach to the project. Thank you.

iwire

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Re: 1200a 3ph 4 wire 208/120v service

The 4 sets of 400 kcmil sound good.

400 kcmil @ 90 = 380 x 4 = 1520 x .8 = 1216 Amps.

Forget the ground wire you do not need it.

Do not forget underground raceways need a wet location conductor.

Bob

bob

Senior Member
Re: 1200a 3ph 4 wire 208/120v service

Bplus
Why do you think the neutral is current carrying?If it only carries the unbalanced load it is not considered current carrying. Is you reason because the lighting has electronic ballast and harmonics?

[ April 21, 2004, 07:43 PM: Message edited by: bob ]

iwire

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Staff member
Re: 1200a 3ph 4 wire 208/120v service

By the way you may not need to count the neutral as a current carrying conductor.

Here is the line from the code:

Part of 310.15(B)(4)(c)
a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads,
In your calculations is the lighting a "major portion of the load".

If not 3 sets of 600 kcmils would do it.

[ April 21, 2004, 07:45 PM: Message edited by: iwire ]

ryan_618

Senior Member
Re: 1200a 3ph 4 wire 208/120v service

Originally posted by iwire:
Do not forget underground raceways need a wet location conductor.

Bob
Also do not forget that if you are derating, most wires with a "W" on their insulation are only good for 75degreeds when installed in a wet location. See table 310.13

This can be a MAJOR issue if the inspector is sharp. I strongly reccomend using a "-2" conductor, such as THWN-2. Refer to note 4 of table 310.13.

bwyllie

Senior Member
Re: 1200a 3ph 4 wire 208/120v service

The THD limits of the ballasts for fluorescent lighting negates the need for oversized neutrals on a 480Y/277V system.

iwire

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Staff member
Re: 1200a 3ph 4 wire 208/120v service

Oversize neutrals are a different issue, than counting the neutral as a current carrying conductor.

That aside it seems unlikely IMO that the lighting load on the service conductors would be a major portion of the load.

Bob

[ April 21, 2004, 08:34 PM: Message edited by: iwire ]

bwyllie

Senior Member
Re: 1200a 3ph 4 wire 208/120v service

true, mis-stated ont he over size neutral, meant considering the neutral as a current carrying conductor.

bplus

Member
Re: 1200a 3ph 4 wire 208/120v service

Thanks Guys!
Good feed back.

shaun

Member
Re: 1200a 3ph 4 wire 208/120v service

iwire's previous message:
"Forget the ground wire you do not need it."

Not questioning your judgement, but for informational purposes, why does he not need a ground in this particular case? Is it becuse he is going underground? I know it maybe a stupid question with an easy answer But I don't know it.

Thanks

Shaun

hurk27

Senior Member
Re: 1200a 3ph 4 wire 208/120v service

By Shaun: why does he not need a ground in this particular case?
Because the neutral is bonded at both the service equipment and at the transformer pad. As allowed by 250.142(1)&(2)

iwire

Moderator
Staff member
Re: 1200a 3ph 4 wire 208/120v service

Originally posted by shaun:
Keep questioning, I make mistakes.

Originally posted by shaun:
but for informational purposes, why does he not need a ground in this particular case?
Take a look at the service for a house, is there a seperate equipment grounding conductor from the power company transformer and the service disconnect?

This post is (at least sounds like) about service conductors from a pad mount transformer to the service disconnect switch inside the building.

As Wayne has pointed out the transformer will have the XO (neutral) bonded to the transformer enclosure. Bonding XO (neutral) of the transformer to the transformer enclosure insures a short circuit if one of the other ungrounded phases was to make contact with the transformer enclosure. That is the power company standard operating procedure.

Inside the building we are required to bond the neutral to the service equipment enclosure and the grounding electrode system.

With both of these enclosures bonded to the neutral any equipment grounding wire run from one enclosure to the other is a redundant and a waste of wire.

Also this equipment grounding conductor will be electrically parallel to the neutral conductor and it will end up carrying part of the neutral current.

This graphic ilistrates this happening between a sevice disconect and a meter, replace the meter with a transformer.

Credit to Ed M. for this graphic.

[ April 22, 2004, 04:59 AM: Message edited by: iwire ]

shaun

Member
Re: 1200a 3ph 4 wire 208/120v service

Thank you guys, you have been a big help!

Shaun

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